* {
    margin: 0;
    padding: 0;
}
.all {
    width: 100%;
    height: 100%;
    position: fixed;
    background-repeat: no-repeat;
    background-size: 100% 100%;
    background-image: url(../resource/壁纸.jpg);
}
.bg {
    width: 100%;
    height: 600px;
    position: relative;
}
.login_div {
    position: absolute;
    left: 500px;
    width: 400px;
    height: 400px;
    margin-top: 200px;
    background-color: grey;
}